It has recently been found in rodents that the circadian rhythms of a mother and her pups are synchronized during perinatal development, and that entrainment of the pups by the mother is likely to begin in utero. This discovery is important not only for its bearing on fundamental question of circadian physiology but also because it defines a potentially important and previously unexplored aspect of maternal-fetal interactions. The rhythms which are entrained by the mother are thought to be regulated by a circadian pacemaker in the central nervous system, the subprachiasmatic nucleus (SCN) of the hypothalamus. Critical to the study of mammalian circadian rhythms is understanding how the SCN generates circadian oscillations, is entrained by external rhythmicity, and regulates many rhythms and possibly many oscillators within the same organism. Prenatal maternal entrainment can be assessed by measuring the postnatal rhythms of pups after separation from the mother. The proposed project will use the activity/rest rhythm of hamsters 18 days after birth as an assay of circadian rhythms, and its goals will to identify when in utero entrainment begins and by what aspect of maternal rhythmicity it is mediated. Neurogenesis of the hamster SNC will also be described, and the possibility that maternal rhythms influence neural proliferation will be investigated. Studying both the functional and structural development of the SCN will advance our understanding of the minimum neural organization necessary for the generation of circadian oscillations. In addition, the interaction between the mother and the fetus provides a unique opportunity to study the pathways and mechanism of entertainment necessary for coupling between oscillators within the same organism. It is increasingly clear that the mother and fetus are components of the same circadian system; the fetus is immersed in and dependent upon a regularly varying environment provivded by the mother, and is, in turn, an integral part of her internal temporal organization. The importance of regular maternal rhythmicity to normal fetal growth and development is not known, but is most likely to be so in animals of long gestation. Disorders related to circadian rhythms in humans, such as depression of long gestation. Disorders related to carcadian roots in rhythmicity of the prenatal environment and its influence on developing fetal physiology.